[SIZE=14pt]It really depends on the bloodlines. Some bloodlines mature fast - all their height by 18 months to 2 years and they won't put on more. Others will put on MOST of their height in the first 18 months and then put on another 1 to 2 inches sometime before they turn 3. Most don't seem to grow much after age 3. [/SIZE]
Best bet is to talk to people who have closely related ponies and find out how quickly they grew and if they put on all their height early or held on put on more later.